Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 23)

Communication Concepts
Chapter 2
2-11
As with reads, writes also are classified by the level of access to PC
memory. Non-physical writes can access only the data table at a remote
PC; physical writes can access all of user memory, including PC program
memory.
Non-physical writes can be further subdivided into protected and
unprotected. Protected writes can access only specified areas of the
remote PC’s data table memory. The accessible areas are defined by
memory protection rungs in the remote PC’s ladder diagram program.
Unprotected writes, on the other hand, can access any area of the remote
PC’s data table.
In most cases, switch settings on the remote station’s interface module can
disable the module from executing each of these types of write
commands.
Diagnostics
Diagnostic commands have to originate from a device other than a PC.
You can use these commands to return status information from a remote
or local station or to alter some on-board parameters at a station interface
module. Diagnostic commands are particularly useful during a start-up or
during on-line debugging.
Mode Select
Mode select commands allow you to load a new PC program from a
remote computer station. The operation of these commands varies by PC
processor type. These commands can be issued only by a computer.
Network Management Layer
The network management layer is concerned with the specifics of
conveying a message safely from its source to its destination. This layer
is the same for both asynchronous and network links.
If your physical link contains only Allen-Bradley PCs, you do not have to
program anything for this layer; the communication interface modules
automatically take care of it. If your physical link contains a computer,
then refer to Chapter 5 for a description of how to program this layer at
the computer station.